Estriol tripropionate (brand name Estriel), or estriol tripropanoate, is semisynthetic, steroidal estrogen. Estriol is a safe estrogen hormone that has been used for decades in Europe to treat hot flashes and vaginal dryness. Estriol is a major urinary estrogen. During pregnancy, large amount of estriol is produced by the placenta. Isomers with inversion of the hydroxyl group or groups are called epiestriol. Though estriol is used as part of the primarily North American phenomenon of bioidentical hormone replacement therapy, it is not approved for use by the FDA or Health Canada. It is however available in the United States by prescription filled only by compounding pharmacies. It has also been approved and marketed throughout Europe and Asia for approximately 40 years for the treatment of post-menopausal hot flashes. It is an estrogen receptor agonist.